This invention relates to a method of degassing a plurality of semiconductor substrates, and to associated apparatus.
The fabrication of semiconductor devices is performed under carefully controlled conditions where it is generally necessary to maintain low levels of impurities. One potential source of impurities is the semiconductor wafer to be processed. Accordingly, it is well known to perform a degassing procedure on a semiconductor wafer prior to one or more fabrication process steps. For example, semiconductor device fabrication processes frequently use techniques for depositing thin layers of high purity materials on to a substrate. It is very important that during these processes, the process chamber is kept free of contaminants so that the deposited film has the required properties. Many standard industry techniques such as PVD and CVD operate under vacuum conditions. It is important that a low pressure is maintained so that minimal contamination of the deposited films occurs.
Many modern production methods require the use of substrates that are coated with materials that may outgas very heavily when exposed to heat. Examples of such materials include organic passivation layers, adhesives, photoresists or spin-on materials. In addition, novel substrate materials are increasingly being used that outgas significantly. Within the wafer packaging industry, materials such as polyimide (PI) and polybenzoxazole (PBO) which are especially problematic. When materials such as these outgas, contaminants are released into the process tool and are incorporated into a growing film. This can cause substantial problems. For example, the properties and characteristics of the deposited films can be degraded.
Cluster tools are commonly used for cost effective manufacture of semiconductor wafers. Typically, wafers are transported from cassettes or Front Opening Unified Pods (FOUPs) into a transport module which operates at a low pressure. One or more robots within the transport module move wafers from the cassettes/FOUPs to numerous modules positioned close to the transport module. The modules which are positioned around the transport module typically operate under vacuum conditions and each have dedicated functions such as degas, etch, PVD deposition amongst other functions.
It is common to degas semiconductor substrates in a dedicated degas process module prior to a subsequent process step such as deposition as part of a standard process flow. It is known to provide degas modules in which a single wafer is degassed at a time. Batch degas modules are also known. In these arrangements, a number of wafers are loaded and degassed together. However, many novel materials used within the semiconductor industry have extremely low rates of degassing which leads to long degas times of 30 minutes or more. However, it is important to achieve a high throughput of any commercial process equipment so that the associated fixed cost can be minimised on a per wafer basis. As a result, it may not be economically viable to process substrates which outgas heavily, because the low degas rates and long degas times can result in a throughput which is too low to be commercially acceptable.